CO₂ Output of Mice Measured by D₂O¹⁸ Under Conditions of Isotope Re-entry Into the Body

¹ From the Department of Physiology, University of Minnesota Medical School, Minneapolis, Minnesota

The fractional turnover rates of the hydrogen and oxygen of the body water of mice were measured in three ways: a) by material intake, b) by material output and c) isotopically. Discrepancies occurred between the intake and output turnover rates which could be explained at least in large part by body weight changes. The isotopic turnover rates were lower than those calculated from material output. This finding is discussed in relationship to the diarrhea which the animals developed on the milk diet employed and to the circumstance that opportunity was present for isotope re-entry from excreta. The difference between the turnover rates of the oxygen and hydrogen of the body water was practically the same whether obtained isotopically or calculated from the material balance data. This probably explains the observation that the D₂O¹⁸ method for calculating the total CO₂ output of the mice from the difference between the isotope turnover rates gave valid results in animals in which the absolute values for the isotopic turnover rates were presumably in error.